1. Field of the Invention
This invention relates generally to power transmission drive belt for a variable-pitch drive, more particularly to a belt for a continuously variable transmission, and specifically to a rubber CVT belt with carbon fiber tensile cord.
2. Description of the Prior Art
Variable-pitch drives (“VPD”) include at least one variable-pitch sheave. Speed ratios on these drives are controlled by moving one sheave sidewall relative to the other so that the belt rides at different pitch diameters in the variable-pitch sheave. A VPD may have two variable-pitch sheaves for a greater range of speed ratios. Such drives have traditionally all been called “variable-speed drives,” but with the development of the closed-loop controlled, continuously variable transmission (“CVT”), the term “variable-speed drive” (“VSD”) has come to be associated more with simpler open-loop controlled drives, in which a speed ratio is selected or dialed in or adjustable, and VSD will be used with that meaning herein. Thus, VSD are variable belt drives with relatively slow, infrequent, or manual speed shifting or ratio adjustment. A CVT, on the other hand, generally has some kind of closed-loop control or feedback mechanism for automatic and relatively rapid shifting based on the dynamics of the drive in a system. Often, in a CVT the driver sheave is controlled based on or reacts to a speed measurement or speed change in order to keep the power source or motor within an optimum power or speed range, and the driven sheave is controlled based on or reacts to the torque load. The variable-pitch sheaves may be adjusted by various mechanisms including mechanical, electro-mechanical, electronic, hydraulic, or the like. Belt-driven CVTs are widely used in scooters, all-terrain vehicles, snowmobiles, and even automobiles. Generally, as two pulley halves move axially apart or together to force a change in belt radial position in any VPD, the belt may be subjected to extreme friction forces as the belt changes radial position within the sheaves. As two sheave halves move together axially to increase the pitch line of the belt, the belt is subjected to extreme friction forces and to high axial or transverse compressive forces. High and variable torque loads result in high tension forces and high wedging forces which also result in high transverse compressive forces on the belt. Some VPD applications also use the belt as a clutch, resulting in additional frictional forces on the contact surfaces of the belt. All these forces may be most severe in a CVT because of the dynamics of the applications (e.g. frequent, rapid shifts, with high acceleration loads). As the CVT belt traverses the driver and driven pulleys, it is also subjected to continual bending or flexing. Rubber CVT belts are generally used without lubrication in so-called “dry CVT” applications. Thus, the VPD belt in general and the CVT belt in particular must be designed to have good longitudinal flexibility, high longitudinal modulus, high abrasion resistance, and high transverse stiffness. The belt must operate across a wide temperature range, for a long time.
Representative of the art is U.S. Pat. No. 6,620,068, which discloses a raw-edge double-cogged V-belt for variable speed drives having curvilinear cogs on the inside and outside, a layer of spirally wrapped cords made of fibers such as polyester, aramid, and/or glass fiber. The belt includes compression and tension layers of rubber containing short fibers aligned laterally for transverse reinforcement. The belt also includes a layer of reinforcing fabric on the inside and/or outside cog surfaces.
Also representative of the art is U.S. Pat. No. 4,708,703, which discloses a CVT belt with aligned upper and lower teeth and grooves, and with longitudinal cords. The teeth are preferably covered at their tops with transverse stiffening elements to deal with the problem of buckling and to increase the torque capability.
U.S. Pat. Nos. 6,695,733 and 6,945,891 to Knutson disclose a toothed rubber belt with carbon fiber tensile cord. Knutson's tensile member is an RFL-treated cord helically spiraled across the width of the belt in spaced side-by-side fashion. The cord examples given are all 6K-2 construction, although 12K-1, 6K-3, and other constructions are also mentioned. The cord may occupy from about 75 to about 95 percent of the belt width. Knutson merely suggests the cord can be used in V-belts among other belt types, but makes no mention of CVT belts.
U.S. Pat. Publ. No. 2001/041636A1 discloses a block type CVT belt comprising an endless load carrier having tensile members with a plurality of rigid blocks mounted thereon. Carbon fiber is mentioned as one of many possible tensile members. This design places no transverse load on the tensile member, but only on the blocks.
U.S. Pat. No. 5,807,194 to Knutson et al. describes carbon fibers, including a 6K-3 construction, for utilization in cast polyurethane toothed belts. The carbon fiber generally possesses a tensile modulus in the range of from about 50 GPa to about 350 GPa as determined in accordance with ASTM D4018. The cross-sectional diameter of the individual carbon fibers is said to be the range of from about 4 to about 7 μm. The carbon cord is not adhesive treated so as not to restrict penetration of the polyurethane materials during casting of the belt. Toothed belts do not operate under transverse loads.
Reference is also made to co-pending U.S. patent application Ser. No. 12/432,985 filed Apr. 30, 2009, and Ser. No. 12/217,026 filed Jul. 1, 2008, the contents of both of which are incorporated herein by reference in their entirety.